Recent investigation shows that, the apparent consumption of diesel fuel in China has already mounted up to about 167 million tons, which leads to frequent occurrence of short supply of diesel fuel (the domestic demand ratio of diesel fuel to petrol is about 2.5:1, but the production ratio thereof is about 2.3:1). Besides the reasons of unreasonable pricing of different types of oil products, and slow price linkage mechanism of domestic petroleum products with international crude oil, the fundamental reason is the constrains of resource shortage. Traditionally, diesel fuel is made from petroleum feedstock, and the resource endowment of China characterized in relatively “rich in coal, poor in oil, and lack in gas” leads to increasingly prominent contradiction between petroleum supply and relatively fast sustainable development of economics and society. Since China became a net importer of petroleum in 1993, the import volume increases fast and constantly, and the foreign-trade dependence already surpassed 56% after 2011, which has a severe impact on national strategic security of energy.
Furthermore, the worsening of crude oil quality leads to continuous scale expansion of domestic catalytic processing of heavy oil and increasing percentage of diesel fuel produced by catalytic processing, which results in gradual decrease of the cetane number (CN value) of diesel fuel products and significant increase of noxious substance discharged after combustion, therefore, the urgent problem to be solved is to increase the CN value of diesel fuel.
The tail gas discharged by a diesel engine contains a large amount of noxious substance such as unburned hydrocarbon compounds and particulate matter (PM), as well as CO, CO2 and NOx, which is one of the main sources of PM2.5 contamination in urban air. International Agency for Research on Cancer (IARC) affiliated to World Health Organization (WHO) declared in June, 2012 the decision to elevate the cancer hazard ranking of diesel engine tail gas, from “possibly carcinogenic” classified in 1988 to “definitely carcinogenic”. As scientific research advances, now there is enough evidence to prove that diesel engine tail gas is one of the reasons that cause people to suffer from lung cancer. Furthermore, there is also limited evidence indicating that, inhaling diesel engine tail gas is relevant to suffering from bladder cancer. People come into contact with diesel engine tail gas through various channels in daily life and work. IARC hopes that this reclassification can provide reference for national governments and other decision makers, so as to actuate them to establish more strict discharge standards of diesel engine tail gas. This significant decision undoubtedly puts forward harsher requirements of diesel fuel quality.
Reducing the content of noxious substance such as sulfur, nitrogen and aromatic hydrocarbon in fuels by petroleum refining process such as hydrogenation is an effective technical route to improve fuel quality, but has very demanding requirements of hydrogenation catalyst and reaction process, with relatively high processing cost. Internationally, many scientific research institutes are carrying out research and development on production technologies of oxygen-containing blending components for petrol and diesel fuel, especially those diesel fuel blending components with high oxygen content and high cetane number, and this has recently become a research hotspot in the technical field of new energy.
Research has indicated that, in consideration of inherent characteristics of oxygen-containing fuel, when oxygen-containing coal-based or methanol-based substance with a high cetane number is added into the fuel as a fuel additive, the discharge of hydrocarbon and CO, especially soot, can be effectively reduced, without changing the original parameters of the engine or increasing the discharge of NOx.
So far, a plenty of researches indicate that, polyoxymethylene dimethyl ethers (abbreviated as PODEn or POMDMEn, n=2-8), which has a general formula of CH3(OCH2)nOCH3 and is a yellow liquid with a high boiling point, an average cetane number reaching above 63 and increasing dramatically as its polymerization degree increases, an average oxygen content of 47%-50%, a flash point of about 65.5° C., and a boiling point of 160-280° C., is a type of clean diesel fuel blending component with a high cetane number, and also is a world-recognized environmental friendly fuel component. Polyoxymethylene dimethyl ethers can be blended into diesel fuel, and can significantly improve the performance of diesel fuel, without the need to modify the engine oil feeding system of the in-use vehicle. However, it is discovered in practical usage that, the cetane number of polyoxymethylene dimethyl ethers is largely influenced by its polymerization degree, and polyoxymethylene dimethyl ethers with a relatively high polymerization degree is required for achieving a better effectiveness. But, in consideration of the inherent difficulty of polymerization reaction, relatively demanding requirements are put forward not only for equipment but also for process conditions, with increased difficulty of processing, separating and purifying. Therefore, people gradually move their focus onto characteristic of polyoxymethylene dialkyl ethers. Polyoxymethylene dialkyl ethers are a series of acetal polymers with low relative molecular weights, which comprise oxymethylene groups as main chain and low carbon alkyl groups as terminal groups, with a general formula of R(OCH2)nOR where R is an alkyl chain of CnH2n+1.
Since the terminal groups of polyoxymethylene dialkyl ethers has relatively high molecular weights in its own, only relatively low polymerization degree is required to achieve a cetane number performance similar to that of polyoxymethylene dimethyl ethers, and the difficulty during the preparation process is relatively low. Polyoxymethylene dialkyl ethers have good environmental protection performance, and when blended into diesel fuel at a certain percentage, they can increase oxygen content of the oil product, and greatly reduce the discharge of contaminants such as SOx, unburned hydrocarbon compounds, PM particulate black smoke and CO from vehicle tail gas. Because polyoxymethylene dialkyl ethers have a high cetane number and physical property similar to that of diesel fuel, they are also a type of diesel fuel additive with very high application value.
Synthesis of polyoxymethylene dialkyl ethers (including polyoxymethylene dimethyl ethers) may be carried out by processing synthesis gas through a series of steps of methanol, formaldehyde, methylal, polyformaldehyde and dimethyl ether etc. China is a famous huge country of coal storage, and Chinese technologies of producing methanol from coal, producing methanol from natural gas and producing methanol from coke-oven gas are increasingly mature, and the production capacity of methanol broke through 50 million tons in 2012, but the equipment operation rate is merely about 50%, thus the problem of methanol surplus has already become very prominent, and the industrial chain of coal chemical industry is in an urgent need to be further extended. Therefore, developing the technology of producing polyoxymethylene dialkyl ethers from coal-based methanol can not only provide a new technology to significantly increase diesel fuel product quality, but also improve the feedstock structure of diesel fuel production, so as to make it more suitable for the resource endowment of domestic fossil energy and enhance the strategic security of domestic supply of liquid fuel for engines.
The preparation process of polyoxymethylene dialkyl ethers should comprise three major process units, wherein, the first unit is a synthesis unit where cascade polymerization reactions and thermodynamic equilibrium reactions catalyzed by acidic catalysts take place; the second unit is a pretreatment unit where processing steps such as deacidifying by neutralization and dehydration by drying take place; and the third unit is a unit for rectification and separation of the downstream products, and this unit attempts to separate polyoxymethylene dialkyl ethers by simple rectification or complicated rectification such as extractive rectification, azeotropic rectification, etc.
So far, domestic and foreign researches on preparation process of polyoxymethylene dialkyl ethers (including polyoxymethylene dimethyl ethers) mainly focus on the aspects of feedstock selection, condition optimization and catalyst system optimization of the synthesis unit, as well as the process technology to improve the distribution of target products and increase product yield. As for optimization of synthesis feedstocks, there are mainly the following five techniques: the first technique is synthesizing polyoxymethylene dimethyl ethers from methanol, formaldehyde, aqueous formaldehyde solution or paraformaldehyde feedstocks, with details described in Patent Applications such as U.S. Pat. No. 6,437,195B2, US2008/0207954A1 and EP1070755A1; the second technique is synthesizing polyoxymethylene dimethyl ethers from methylal, trioxane or paraformaldehyde feedstock, with details described in Patent Applications such as US2007/0260094A1 and US2449469A; the third technique is synthesizing polyoxymethylene dimethyl ethers from methanol and dimethyl ether feedstocks, with details described in Patent Application such as U.S. Pat. No. 6,265,528B1; the fourth technique is developed on the basis of the foregoing three techniques, and this technique uses alcohol-containing by-products of other chemical processes in the prior art to synthesize mixture of polyoxymethylene dialkyl ethers with various polymerization degree and various terminal groups, and the major representative techniques are synthesis of polyoxymethylene dialkyl ethers with various polymerization degree and various terminal groups from the feedstock of industrial alcohol brewing by-products or Fischer-Tropsch synthesis by-products or C4, C5 fractions of petroleum, which are disclosed in Chinese Patent Applications CN102173984A and CN102180778A.
In the above-mentioned technical solutions of synthesis of polyoxymethylene dialkyl ethers, the separation and purification of synthesized products is carried out without exception by conventional ordinary rectification, extractive rectification or azeotropic rectification of the prior art, and no further in-depth research is done in respect of the separation and purification unit of target products. However, it is discovered in practical research that, when separating and purifying target products by using the foregoing conventional seemingly viable means, it always leads to that the separation and purification efficiency of products is not high, and the purity of the separated products is not satisfactory, which is not enough to meet the technical standard for blending with fossil diesel fuel and requires subsequent additional purification operations to meet the needs, and no matter how the parameters and conditions of the entire operating process of the separation and purification unit are optimized, the difficult problem about separation and purification efficiency always cannot be solved, and no significant increase in separation and purification efficiency or product purity can be achieved. In practical production, in consideration of economical and various other aspects, no matter how great the efficiency of the synthesis unit at the front is, the incapability of obtaining required products by effective separating and purifying means is always a difficult problem and bottleneck that restrains the development of this technology, which is an urgent matter to be solved in this field.